The US3 Kinase of Herpes Simplex Virus Phosphorylates the RNA Sensor RIG-I To Suppress Innate Immunity

Cell Intrinsic Determinants of Alpha Herpesvirus Latency and Pathogenesis in the Nervous System

Alpha herpesvirus infections (α-HVs) are widespread, affecting more than 70% of the adult human population. Typically, the infections start in the mucosal epithelia, from which the viral particles invade the axons of the peripheral nervous system. In the nuclei of the peripheral ganglia, α-HVs establish a lifelong latency and eventually undergo multiple reactivation cycles. Upon reactivation, viral progeny can move into the nerves, back out toward the periphery where they entered the organism, or they can move toward the central nervous system (CNS). This latency-reactivation cycle is remarkably well controlled by the intricate actions of the intrinsic and innate immune responses of the host, and finely counteracted by the viral proteins in an effort to co-exist in the population. If this yin-yang- or Nash-equilibrium-like balance state is broken due to immune suppression or genetic mutations in the host response factors particularly in the CNS, or the presence of other pathogenic stimuli, α-HV reactivations might lead to life-threatening pathologies. In this review, we will summarize the molecular virus-host interactions starting from mucosal epithelia infections leading to the establishment of latency in the PNS and to possible CNS invasion by α-HVs, highlighting the pathologies associated with uncontrolled virus replication in the NS.

Viral evasion of the interferon response at a glance

Re-emerging and new viral pathogens have caused significant morbidity and mortality around the world, as evidenced by the recent monkeypox, Ebola and Zika virus outbreaks and the ongoing COVID-19 pandemic. Successful viral infection relies on tactical viral strategies to derail or antagonize host innate immune defenses, in particular the production of type I interferons (IFNs) by infected cells. Viruses can thwart intracellular sensing systems that elicit IFN gene expression (that is, RIG-I-like receptors and the cGAS-STING axis) or obstruct signaling elicited by IFNs. In this Cell Science at a Glance article and the accompanying poster, we review the current knowledge about the major mechanisms employed by viruses to inhibit the activity of intracellular pattern-recognition receptors and their downstream signaling cascades leading to IFN-based antiviral host defenses. Advancing our understanding of viral immune evasion might spur unprecedented opportunities to develop new antiviral compounds or vaccines to prevent viral infectious diseases.

The RNA polymerase III-RIG-I axis in antiviral immunity and inflammation

Nucleic acid sensors survey subcellular compartments for atypical or mislocalized RNA or DNA, ultimately triggering innate immune responses. Retinoic acid-inducible gene-I (RIG-I) is part of the family of cytoplasmic RNA receptors that can detect viruses. A growing literature demonstrates that mammalian RNA polymerase III (Pol III) transcribes certain viral or cellular DNA sequences into immunostimulatory RIG-I ligands, which elicits antiviral or inflammatory responses. Dysregulation of the Pol III-RIG-I sensing axis can lead to human diseases including severe viral infection outcomes, autoimmunity, and tumor progression. Here, we summarize the newly emerging role of viral and host-derived Pol III transcripts in immunity and also highlight recent advances in understanding how mammalian cells prevent unwanted immune activation by these RNAs to maintain homeostasis.

Guanylate-binding protein 1 inhibits nuclear delivery of pseudorabies virus by disrupting structure of actin filaments

The alphaherpesvirus pseudorabies virus (PRV) is the causative agent of pseudorabies, responsible for severe economic losses to the swine industry worldwide. The interferon-inducible GTPase guanylate-binding protein 1 (GBP1) exhibits antiviral immunity. Our findings show that there is a robust upregulation in the expression of porcine GBP1 during PRV infection. GBP1 knockout promotes PRV infection, while GBP1 overexpression restricts it. Importantly, we found that GBP1 impeded the normal structure of actin filaments in a GTPase-dependent manner, preventing PRV virions from reaching the nucleus. We also discovered that viral US3 protein bound GBP1 to interfere with its GTPase activity. Finally, the interaction between US3 and GBP1 requires US3 serine/threonine kinase activity sites and the GTPase domain (aa 1 to 308) of GBP1. Taken together, this study offers fresh perspectives on how PRV manipulates the host's antiviral immune system.

Exploiting RIG-I-like receptor pathway for cancer immunotherapy

RIG-I-like receptors (RLRs) are intracellular pattern recognition receptors that detect viral or bacterial infection and induce host innate immune responses. The RLRs family comprises retinoic acid-inducible gene 1 (RIG-I), melanoma differentiation-associated gene 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2) that have distinctive features. These receptors not only recognize RNA intermediates from viruses and bacteria, but also interact with endogenous RNA such as the mislocalized mitochondrial RNA, the aberrantly reactivated repetitive or transposable elements in the human genome. Evasion of RLRs-mediated immune response may lead to sustained infection, defective host immunity and carcinogenesis. Therapeutic targeting RLRs may not only provoke anti-infection effects, but also induce anticancer immunity or sensitize "immune-cold" tumors to immune checkpoint blockade. In this review, we summarize the current knowledge of RLRs signaling and discuss the rationale for therapeutic targeting RLRs in cancer. We describe how RLRs can be activated by synthetic RNA, oncolytic viruses, viral mimicry and radio-chemotherapy, and how the RNA agonists of RLRs can be systemically delivered in vivo. The integration of RLRs agonism with RNA interference or CAR-T cells provides new dimensions that complement cancer immunotherapy. Moreover, we update the progress of recent clinical trials for cancer therapy involving RLRs activation and immune modulation. Further studies of the mechanisms underlying RLRs signaling will shed new light on the development of cancer therapeutics. Manipulation of RLRs signaling represents an opportunity for clinically relevant cancer therapy. Addressing the challenges in this field will help develop future generations of cancer immunotherapy.

Actin cytoskeleton remodeling primes RIG-I-like receptor activation

The current dogma of RNA-mediated innate immunity is that sensing of immunostimulatory RNA ligands is sufficient for the activation of intracellular sensors and induction of interferon (IFN) responses. Here, we report that actin cytoskeleton disturbance primes RIG-I-like receptor (RLR) activation. Actin cytoskeleton rearrangement induced by virus infection or commonly used reagents to intracellularly deliver RNA triggers the relocalization of PPP1R12C, a regulatory subunit of the protein phosphatase-1 (PP1), from filamentous actin to cytoplasmic RLRs. This allows dephosphorylation-mediated RLR priming and, together with the RNA agonist, induces effective RLR downstream signaling. Genetic ablation of PPP1R12C impairs antiviral responses and enhances susceptibility to infection with several RNA viruses including SARS-CoV-2, influenza virus, picornavirus, and vesicular stomatitis virus. Our work identifies actin cytoskeleton disturbance as a priming signal for RLR-mediated innate immunity, which may open avenues for antiviral or adjuvant design.

Herpes Simplex Virus 1 (HSV-1) Reactivation in Critically Ill COVID-19 Patients: A Brief Narrative Review

Systemic or pulmonary reactivations of herpes simplex virus 1 (HSV-1) have been reported in critically ill patients with COVID-19, posing a dilemma for clinicians in terms of their diagnostic and clinical relevance. Prevalence of HSV-1 reactivation may be as high as > 40% in this population, but with large heterogeneity across studies, likely reflecting the different samples and/or cut-offs for defining reactivation. There is frequently agreement on the clinical significance of HSV-1 reactivation in the presence of severe manifestations clearly attributable to the virus. However, the clinical implications of HSV-1 reactivations in the absence of manifest signs and symptoms remain controversial. Our review aims at providing immunological background and at reviewing clinical findings on HSV-1 reactivations in critically ill patients with COVID-19.

RIG-I Promotes Cell Viability, Colony Formation, and Glucose Metabolism and Inhibits Cell Apoptosis in Colorectal Cancer by NF-κB Signaling Pathway

Background

Retinoic acid-inducible gene-I (RIG-I) has crucial effects on various cancers, while RIG-I's detailed roles and mechanism in colorectal cancer (CRC) are uncovered.

Methods

qRT-PCR was used to detect the expression of RIG-I in CRC, adjacent nontumor specimens, and five cell lines. CCK-8, colony formation, and flow cytometry assays were conducted to study CRC cell viabilities. Extracellular acidification rates, lactate analysis, and ATP analysis were conducted to study the cell viabilities and glucose metabolism of CRC cells. Western blot is used to determine the proteins of NF-κBp65 in the nucleus and cytoplasm.

Results

This study revealed the upregulation of RIG-I in CRC tissues and cells and that high RIG-I expression was correlated with poor prognosis of CRC patients. In addition, silencing RIG-I inhibited cell viability as well as colony formation and promoted cell apoptosis in CRC cells, while RIG-I knockdown suppressed transplanted tumor growth and facilitated apoptosis in nude mice. Moreover, silencing RIG-I inhibited glucose metabolism by decreasing extracellular acidification rate, lactate production, adenosine triphosphate, and content of hypoxia-inducible factor 1α and pyruvate kinase isoform. 2.2-Deoxy-d-glucose, a glycolysis inhibitor, reduced the growth of CRC cells and promoted apoptosis in vitro and in vivo. In addition, RIG-I knockdown decreased NF-κB nuclear translocation. Besides, inhibiting NF-κB effectively eliminated RIG-I overexpression roles in cell viability and glucose metabolism in CRC cells.

Conclusion

In summary, this study revealed that RIG-I mediated CRC cell proliferation, apoptosis, and glucose metabolism at least partly by NF-κB signaling pathway.